1. Field of the Invention
The present invention relates to a liquid crystal display (LCD) device and a method for designing the same, and more particularly, to a trans-reflective type In-Plane Switching (IPS)-LCD device capable of implementing a single gap and a wide viewing angle, and minimizing an occurrence area of disclination at an interface between a transmission region and a reflection region, and a method for designing the same.
2. Description of the Background Art
Nowadays, liquid crystal display (LCD) devices are being widely used due to their advantageous characteristics such as light weight, thin thickness, and small power consumption. Accordingly, the LCD devices are being widely applied to portable electronic devices such as a notebook and a portable phone.
The LCD device displays desired images on a screen by controlling an amount of optical transmissivity according to image signals applied to a plurality of control switching devices arranged in a matrix form.
The LCD device comprises a color filter substrate that is an upper substrate, a thin film transistor (TFT) array substrate that is a lower substrate, an LC panel having an LC layer and disposed between the color filter substrate and the TFT array substrate, and an LC panel driving unit for driving the LC panel by supplying scan signals and image information to the LC panel.
A cathode ray tube (CRT) or a light emitting diode (LED) are spontaneous light emitting device, whereas the LCD device is a non-spontaneous light emitting device. Accordingly, for image implementation, it is necessary for the LCD device to be provided with a light source to supply light to the LC panel.
The LCD device may be largely classified into two types. One is a transmission type using a light source disposed inside the LCD device, and the other is a reflection type using an external light source (e.g., sunlight).
However, the transmission type LCD device requires large power consumption due to an internal light source such as a portable battery. Accordingly, when the transmission type LCD device is applied to portable electronic devices, a long usage time is not obtained due to limitations of a capacity of the portable battery. Accordingly, advantages of a portable characteristic can not be implemented. Furthermore, when the reflection type LCD device is utilized at an indoor room, an external light source such as sunlight is weak to cause a dark brightness and thereby to degrade a screen quality.
To solve these problems, has been proposed a trans-reflective type electrically controlled birefringence (ECB) LCD device that combines the transmission type LCD and the reflection type LCD with each other. The trans-reflective type ECB-LCD device can be easily applied to portable electronic devices due to its small power consumption, and can provide a high quality screen not only indoors but also outdoors.
Hereinafter, the trans-reflective type ECB-LCD device will be explained in more detail with reference to the attached drawings.
FIG. 1 shows a general trans-reflective type ECB-LCD device in accordance with the conventional art.
Referring to FIG. 1, the trans-reflective type ECB-LCD device comprises a first substrate 1 and a second substrate 2 each having a transmission region and a reflection region; at least one of phase compensation plates 16a and 16b, and a lower polarizer 3 below the first substrate 1; and at least one of phase compensation plates 17a and 17b, and an upper polarizer 4 above the second substrate 2. A first pixel electrode 5a and a second pixel electrode 5b for applying an electric field to an LC layer 8 are formed on the first substrate 1 in a transmission region and a reflection region, respectively. The first pixel electrode 5a and the second pixel electrode 5b are electrically connected to each other. A color filter 20 and a common electrode 6 are formed on the second substrate 2. The common electrode 6 forms an electric field together with the first and second pixel electrodes 5a and 5b on the first substrate 1, thereby driving the LC layer 8.
The first pixel electrode 5a in the transmission region is formed of a transparent conductive material having comparatively high optical transmissivity, thereby transmitting light supplied from an internal light source and easily emitting the light outside the LCD device. On the contrary, the second pixel electrode 5b in the reflection region is formed of an opaque conductive metallic material having comparatively high reflectivity, thereby easily reflecting external light incident from outside.
In the reflection region of the conventional trans-reflective type ECB-LCD device, light incident from an external light source passes through the LC layer two times. However, in the transmission region, light supplied from an internal light source passes through the LC layer one time. In order to compensate different optical characteristics between the reflection region and the transmission region, the conventional trans-reflective type ECB-LCD device has a dual cell gap structure that a cell gap (d) in the transmission region is two times a cell gap (d/2) in the reflection region.
However, the trans-reflective type ECB-LCD device having a dual cell gap structure does not have simplified fabrication processes due to a stepped portion between the reflection region and the transmission region, thereby causing low productivity.
Furthermore, in the trans-reflective type ECB-LCD device, LC molecules are driven by a vertical electric field formed by the first and second pixel electrodes on the first substrate, and the common electrode on the second substrate. This may cause a narrow viewing angle.